feat: 实现对外邮件投递（外发队列 + MX 直投 + DKIM + 退信 + 管理后台）

- 新增 internal/outbound 模块：MX 查询、SMTP 出站客户端（EHLO/STARTTLS/
  MAIL/RCPT/DATA/QUIT）、4xx 临时失败与 5xx 永久失败分类、8BITMIME 支持
- 新增 outbound_messages 队列表与 OutboundStore，后台 worker 指数退避重试
- 永久失败/超限退信到发件人收件箱，包含原因与目标收件人
- 外发邮件使用域名 DKIM 私钥签名（go-msgauth）
- SMTP 提交集成：认证用户可发外部收件人，MAIL FROM 必须等于登录用户邮箱，
  未认证外部投递明确拒绝（防开放中继）
- Web 发信集成：外部收件人自动入队，附件以 multipart/mixed + base64 编码
  加入邮件正文
- 每用户每分钟/每日发送限速（max_per_day=0 可禁用外部投递）
- 管理后台新增外发队列页面：状态统计、失败原因、手动重试/取消
- 新增 [outbound] 配置段并更新 README / todo.md

internal/store/outbound_store.go

@@ -0,0 +1,100 @@
+package store
+
+import (
+	"time"
+
+	"mail_go/internal/db"
+
+	"gorm.io/gorm"
+)
+
+// OutboundStore defines the interface for outbound queue operations.
+type OutboundStore interface {
+	Create(msg *db.OutboundMessage) error
+	GetByID(id uint) (*db.OutboundMessage, error)
+	ListDue(now time.Time, limit int) ([]db.OutboundMessage, error)
+	List(page, size int, status string) ([]db.OutboundMessage, int64, error)
+	Update(msg *db.OutboundMessage) error
+	Delete(id uint) error
+	CountByStatus(status string) (int64, error)
+}
+
+// outboundStoreGorm implements OutboundStore using GORM.
+type outboundStoreGorm struct {
+	db *gorm.DB
+}
+
+// newOutboundStore creates a new GORM-backed OutboundStore.
+func newOutboundStore(database *gorm.DB) OutboundStore {
+	return &outboundStoreGorm{db: database}
+}
+
+// Create inserts a new outbound queue record.
+func (s *outboundStoreGorm) Create(msg *db.OutboundMessage) error {
+	return s.db.Create(msg).Error
+}
+
+// GetByID retrieves an outbound queue record by primary key.
+func (s *outboundStoreGorm) GetByID(id uint) (*db.OutboundMessage, error) {
+	var msg db.OutboundMessage
+	if err := s.db.First(&msg, id).Error; err != nil {
+		return nil, err
+	}
+	return &msg, nil
+}
+
+// ListDue retrieves messages that are due for a delivery attempt.
+func (s *outboundStoreGorm) ListDue(now time.Time, limit int) ([]db.OutboundMessage, error) {
+	var msgs []db.OutboundMessage
+	if err := s.db.
+		Where("status IN (?, ?) AND next_attempt_at <= ?", db.OutboundStatusPending, db.OutboundStatusDeferred, now).
+		Order("next_attempt_at ASC").
+		Limit(limit).
+		Find(&msgs).Error; err != nil {
+		return nil, err
+	}
+	return msgs, nil
+}
+
+// List retrieves a paginated list of outbound messages, optionally filtered by status.
+func (s *outboundStoreGorm) List(page, size int, status string) ([]db.OutboundMessage, int64, error) {
+	var msgs []db.OutboundMessage
+	var total int64
+
+	query := s.db.Model(&db.OutboundMessage{})
+	if status != "" {
+		query = query.Where("status = ?", status)
+	}
+	if err := query.Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	offset := (page - 1) * size
+	if status != "" {
+		err := s.db.Where("status = ?", status).Order("id DESC").Offset(offset).Limit(size).Find(&msgs).Error
+		return msgs, total, err
+	}
+	if err := s.db.Order("id DESC").Offset(offset).Limit(size).Find(&msgs).Error; err != nil {
+		return nil, 0, err
+	}
+	return msgs, total, nil
+}
+
+// Update saves changes to an existing outbound queue record.
+func (s *outboundStoreGorm) Update(msg *db.OutboundMessage) error {
+	return s.db.Save(msg).Error
+}
+
+// Delete removes an outbound queue record by ID.
+func (s *outboundStoreGorm) Delete(id uint) error {
+	return s.db.Delete(&db.OutboundMessage{}, id).Error
+}
+
+// CountByStatus returns the number of outbound messages in a given status.
+func (s *outboundStoreGorm) CountByStatus(status string) (int64, error) {
+	var count int64
+	if err := s.db.Model(&db.OutboundMessage{}).Where("status = ?", status).Count(&count).Error; err != nil {
+		return 0, err
+	}
+	return count, nil
+}

internal/store/stores.go

@@ -13,6 +13,7 @@ type Stores struct {
 	Domains     DomainStore
 	Attachments AttachmentStore
 	Bans        BanStore
+	Outbound    OutboundStore
 }
 
 // NewStores creates a new Stores instance with all GORM-backed implementations.
@@ -23,6 +24,7 @@ func NewStores(database *gorm.DB) *Stores {
 		Domains:     newDomainStore(database),
 		Attachments: newAttachmentStore(database),
 		Bans:        newBanStore(database),
+		Outbound:    newOutboundStore(database),
 	}
 }